Have you tried to reset to the original FairPhone OS to find out if the problems are on the hardware side?
(Please be very cautious with changing OS, since there are current problems with Fairphones and the rollback protection - leading to final brick of the devices. See the many, many discussions about FairPhones here in the forum - you can find them by using the forum key word “Fairphone4”, showing you all the threads about FP4. One example: FP4 - Unable to relock bootloader - #35 by rik ).